Add key+=append syntax to sysrc(8).

Reviewed by:	shurd
MFC after:	3 days
X-MFC-to:	stable/10 stable/9


git-svn-id: svn+ssh://svn.freebsd.org/base/head@274068 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f